Alcator C-Mod Weekly Highlights June 14, 2005 FY2005 weeks of research operations Planned: 17 weeks Completed: 8.9 weeks Operations ---------- Alcator C-Mod operated 3.5 days last week. On Wednesday and Thursday discharges were run in helium and deuterium, and in upper and lower null configurations to study performance aspects of the ICRF antennas. The machine was boronized on Thursday evening, and a very successful run documenting post boronization plasma performance was performed on Friday. Friday's run was extended to 9:00 PM so that time was available to see a change in performance as the effects of the boron varied. A brief vent of air through the high pressure disruption mitigation gas jet primary gate-valve required a short bake and electron cyclotron discharge cleaning for cleanup on Monday afternoon and Tuesday. Plasma operation will continue this week. Physics ------- On Wednesday we ran both deuterium and helium discharges as we began to investigate ICRF antenna performance in the two types of discharges. The helium discharges had significant deuterium remaining from previous operation, but preliminary analysis indicated that D- and E-Port antenna operation was improved in helium. ECDC was run in helium Wednesday night to accelerate removal of deuterium from the walls in preparation for Thursday's run. On Thursday a significant improvement in the D-Port antenna operation in helium was clear. With a switch back to deuterium plasmas, the antenna performance decreased but was still improved over previous operation in deuterium. The D-Port antenna reached 0.75 MW for the first time since the vent to remove the lower hybrid launcher. The E-Port antenna also ran well at up to 45 kV in helium plasmas and showed slightly increased faulting with the switch back to deuterium. We were unable to reach high enough neutral pressures in helium discharges to determine if the neutral pressure limit was different from deuterium plasmas. This run confirmed that the J-Port antenna could couple into high density plasmas without reliability issues when the neutral pressure remained low. Boronization of the machine was carried out on Thursday evening into early Friday morning. Dramatic drops in moly and titanium levels were noted on Friday as well as a factor of two drop in the H/(H+D). The ECDC radial scan was kept well away from the inner wall with a 20 s dwell near the limiter/antenna surfaces. It is becoming clear that boronization of the inner wall is not a necessary ingredient in achieving good machine performance. This conclusion is consistent with measurements of boron deposition on the inner wall and inner divertor made by Dennis Whyte, UW. Friday's run was very successful, with a total for 48 shots, of which 30 had at least 3 MW of ICRF power. Performance peaked after about 20 discharges with a stored energy of 210 kJ at 4 MW and 170 kJ at 3 MW. Performance at these high levels lasted for about 8 discharges at 4 MW and about 15 discharges at 3 MW, when a definite deterioration was observed. We are investigating several new techniques by which the excellent performance following boronization can be extended. Lower Hybrid ------------ A second stainless steel 3-window prototype braze was completed last week with very positive results. The prototype is leak-tight with no cracks in the ceramic and very uniform braze flow. We are proceeding with a flanged 3-window prototype. Work continues on the simulation and design of water loads for the lower hybrid launcher. These loads will absorb power reflected from the couplers. ICRF Systems ------------ Work continued on improvements to the transmitter water cooling system, and modifications expected to improve FPA tube reliability. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly